The present invention relates to a rubber composition, in which reversion is inhibited without losing rubber properties such as scorch properties, and a pneumatic tire using the composition.
Conventionally, sulfur has mainly been used as a vulcanizing agent for vulcanizing rubber. Usually, when diene rubber such as isoprene rubber is vulcanized at a high temperature using sulfur as a vulcanizing agent, as shown in FIG. 1, polysulfide bonds that link the crosslinking points are split and reversion occurs. When reversion occurs, there is the problem that properties, such as elastic modulus and heat generating properties, of the vulcanized article decrease. Also, there is the problem that rubber properties such as strength at break and elongation at break decrease, as the polysulfide bonds in the rubber also tend to break easily after vulcanization. To solve these problems, employed are the method of vulcanizing at a relatively low temperature and methods such as semi EV vulcanization (semi-effective vulcanization) and EV vulcanization (effective vulcanization), in which sulfur vulcanization reaction is conducted efficiently by reducing the amount of sulfur and increasing the amount of the vulcanization accelerator. However, as secondary effects of these methods, a long period of time is required for vulcanization, thereby decreasing productivity, and there are problems in processability, as scorch properties are lost.
Also, known is the method of using a vulcanizing agent for rubber that inhibits reversion when vulcanizing (see JP-A-2002-53706). However, both the effects of inhibiting reversion and preventing decrease in rubber properties cannot be satisfied.
Furthermore, known is a rubber composition in which expanded graphite is compounded in diene rubber, but the effect of inhibiting reversion is not satisfactory (see JP-A-2002-293990).
The present invention aims to provide a rubber composition, in which reversion when vulcanizing at a high temperature is inhibited without losing rubber properties and which is obtained by efficiently crosslinking with sulfur, and a pneumatic tire using the rubber composition.